1. Field of the Invention
This invention relates to an electron tube cathode used in television tube, etc., particularly to an electron tube cathode having a short rise time for reaching an actuation state, and more particularly to an electron tube cathode having a base metal consisting of a nickel-based alloy containing tungsten in solid solution approximately up to its solubility limit (20-28% by weight of tungsten) to enhance a high temperature strength, and furthermore to a method for producing these electron tube cathodes.
2. Brief Description of the Prior Art
Television tubes now used are mostly of an indirectly heated tube, using a base metal consisting of a Ni-0.05% Mg alloy or a Ni-(2-4)% W alloy, each alloy further containing a reducing impurity. This type of heating utilizes a heated radiation of a tungsten filament inserted in a cathode sleeve. Thus, in the conventional television tube, it takes about 30 seconds to make a normal picture on screen. To improve this disadvantage, the tungsten filament in the indirectly heated type cathode is made finer, and furthermore to improve a radiation efficiency, a tungsten heater surface is subjected to a black treatment. As a result, the rise time can be made as short as about 5 seconds. However, in order to produce an instantaneous picture of much shorter duration, it is necessary to decrease a heat capacity. That is, the thickness of base metal must be greatly reduced from the presently prevailing size of 100-150 .mu.m. However, the reduction of the thickness of base metal has the following problems in the conventional cathode metal composition:
(i) Content of reducing impurity is decreased, and consequently an emission life is shortened.
(ii) High temperature strength is lowered, and thus a heat stress generated by reaction between oxides constituting an electron-emitting material and the base metal cannot be cancelled, causing a heat deformation to lose a white balance between R.G.B guns.
A high temperature strength can be obtained by adding tungsten to nickel approximately up to its solubility limit. Even in a directly heated type cathode, in which the cathode metal is heated by direct passage of electric current, a high temperature strength and a high specific resistance are required. To this end, tungsten is usually added to nickel approximately up to its solubility limit.
The cathode metal of such a composition has distinguished mechanical and electrical properties, but is not practically utilized owing to an unstableness of its emission characteristics. That is, a tungsten interface layer (a reaction product from tungsten oxide and alkaline earth metal oxide, formed at a boundary between the base metal and the alkaline earth metal oxide coating) is extremely increased in a cathode metal having a tungsten content as large as 20-28% by weight, while the tungsten interface layer is not a serious problem in the ordinary cathode metal having a small tungsten content, for example, 2-4% by weight. Such an extreme increase in the tungsten interface layer consequently causes to peel the oxide coating off, and further causes to deteriorate or fluctuate emission characteristics, making the cathode metal unreliable.
In the case of directly heated type, the base metal must be as thin as 30-40 .mu.m, and thus the content of the reducing impurity must be about 5 times as large as that of the indirectly heated type having 150 .mu.m thickness. For example, when zirconium, which is well-known for a high diffusion speed as the reducing impurity, is added to a cathode metal, the necessary amount of zirconium to be added is 0.3-0.5% by weight, and Ni-28 wt% W-0.4 wt% Zr can be given as one example of alloy composition for the directly heated type cathode metal.
However, the Ni-based alloy for the cathode metal so far practically employed contains 2-3% by weight of tungsten, and it has been said that it is impossible to use a Ni-based alloy containing 20-28% by weight of tungsten as the cathode metal on account of the peeling of the oxide coating and deterioration of emission characteristics.
The following references are cited to show the state of the art; (i) U.S. Pat. No. 2,833,647, (ii) U.S. Pat. No. 3,374,385, and (iii) Japanese Patent Publication No. 6,918/61.